Optional preferential voting
Optional Preferential Voting (OPV) is a system of vote-casting used in the states of Queensland and New South Wales in the Commonwealth of Australia. Most Australian elections are run under full-preference preferential voting, where all candidates must be numbered in order of the preference of the voter, or the vote will not be counted. Although complete numbering is not required under the OPV system, a single-preference vote must still use a '1' and not a tick or cross in order for the cast vote to be considered valid.
In other countries, such as Malta, the Republic of Ireland, and Northern Ireland, full preferences are not required.
Official description
As described by Electoral Commission Queensland:
Queensland's voting system - optional preferential voting
Queensland State elections have used Optional Preferential Voting since the 1992 State election. OPV is also used in Queensland local government elections (in those councils divided into single member wards or divisions) and in the New South Wales Lower House.
OPV provides 3 options for voters’ so that a valid vote may be cast by:
1. expressing a single primary preference for one candidate only, leaving all other squares blank (this is called ‘plumping’ for one candidate)
2. expressing a partial distribution of preferences by voting for some, but not all candidates on the ballot paper (for example, voting 1, 2, 3 on a ballot with five candidates)
3. expressing a full distribution of preferences (that is, marking every square in order of preference).
Practical usage
This form of voting allows for one single candidate or candidates of similar ideology to be endorsed by a voter rather than directing preferences to all candidates.
This has been the cause of some concern due to the use of the 'Just vote 1' system by Queenslanders in federal elections, where there is no optional preferential voting, and led to many votes (likely to be ALP votes due to their limited affiliation with minor parties) not being counted.